Re: Failing Test Cases in HEAD

`devel2` compiles GHC with extra assertions and `./validate` doesn't
do that. There's `./validate --slow` that enables extra assertions.
AFAIK harbormaster (and most ghc-devs) only does `./validate` which
explains how failures like this can sneak in.

We used to have a travis build that did `./validate --slow`, but it
looks like travis currently fails at configure with:

configure: error: GHC version 8.0 or later is required to compile GHC.